Prevention of population cycles by parasite removal

The regular cyclic fluctuations in vertebrate numbers have intrigued scient ists for more than 70 years, and yet the cause of such cycles has not been clearly demonstrated. Red grouse populations in Britain exhibit cyclic fluc tuations in abundance, with periodic crashes. The hypothesis that these flu ctuations are caused by the impact of a nematode parasite on host fecundity was tested by experimentally reducing parasite burdens in grouse. Treatmen t of the grouse population prevented population crashes, demonstrating that parasites were the cause of the cyclic fluctuations.